The invention relates to the driving of picture tubes, in particular of colour picture tubes used in television receivers or monitors.
The colour picture tubes used in contemporary colour television sets are predominantly controlled via the cathode of the respective beam current system, the maximum possible luminous efficiency being determined by the respectively available modulation range. The control electrodes designed as a Wehnelt electrode are usually connected to reference-earth potential (earth). In order to obtain a high luminous efficiency, variables which limit the modulation range, such as e.g. maximum supply voltage of the RGB output stages, saturation of the RGB output stages, cut-off regulating range, must be carefully taken into account during the circuit design.
For the colourless reproduction of black and white pictures, it is customary in contemporary colour television sets to provide a cut-off regulation for each colour channel. In this case, a so-called cut-off measurement line is respectively keyed in during a vertical blanking interval, to which line a reference cathode current I.sub.k1 of e.g. 10 .mu.A corresponds. Cut-off regulation of this type is known e.g. from the video processor circuit TDA4580 from Valvo.
Tubes go through various phases during their ageing process. At the beginning, roughening and, as it were, blooming of the cathode surfaces frequently occur.
These reduce the effective spacing between cathode and Wehnelt electrode. If the operating voltages are kept constant, the field strength between the electrodes therefore increases and, consequenty, the cathode current decreases. With increasing age, more and more cathode material is eroded, with the result that the effect is reversed. The cut-off regulation reacts to the rise in the field strength described first in a manner such that the reverse voltage acting between cathode and Wehnelt electrode is correspondingly reduced in order to be able to drive the same reference cathode current I.sub.k1. However, the reaction of the cut-off regulation must be taken into account for the circuit design since this results in a reduction in the modulation range, which, in turn, has an adverse effect on the maximum luminous efficiency that can be achieved.